The invention relates to a process for joining vacuum heat insulating elements which are fabricated in various shapes, including the shape of a pipe, a cylinder or a dome.
Vacuum heat insulating elements are hollow bodies comprised of substantially parallel surfaces, usually made of metal sheets, which are arranged at an approximately uniform separation from each other, and between which a vacuum is maintained, so as to inhibit the flow of thermal energy transversely between the surfaces, that is between the surface which is warmer and the surface which is colder. The hollow space is filled with a pressure-resistant powder which is a poor conductor of heat, such as siliceous earth, and is evacuated to a low pressure value, for example 10.sup.-3 bar. As the spacing device, the powder filling absorbs the pressure load caused by the atmospheric pressure or by the respective operational pressure (pressure pipes, pressure containers, etc.) and reduces radiant heat transmission. In the edge area, the two surfaces or walls of such elements are connected by thin-walled diaphragms corrugated in an essentially rippled shape and consisting of a material with a relatively low thermal conductivity (such as stainless steel) in order to reduce the heat transmission by conduction.
For specific applications, such as pipe lines and containers, it is useful to manufacture the heat insulating elements in the form of one-dimensionally or two-dimensionally curved surfaces with axially aligned, surrounding edges. Thus, a container, for example, may be made by connecting a hemispherical or dome shaped heat insulating element, a ring shaped or cylindrical element and another dome-shaped heat insulating element. Because of the fissured surfaces of the vacuum heat insulating elements caused by connection of the elements, surrounding gaps or hollow spaces (at the joints) exist which are particularly critical with respect to heat losses. The simplest--and least desirable--construction consists of connecting only the outside or inside walls of the vacuum heat insulating elements tightly with one another and leaving the gaps at the joint open on one side in the direction of the enclosed medium or the environment (generally: air). Thus, only a slight insulating effect exists in the joint area so that relatively high heat losses must be expected. (The space between the surfaces of the respective insulating elements themselves remains sealed, of course, by diaphragm type seals at the edges, as describe above.)
A substantial improvement can be achieved filling the gaps with insulating material (such as mineral wool, foamed material). The problems in this arrangement result from the resistance to temperature changes and the ageing properties of such materials, as well as the absorption of moisture and liquid. Also, the insulating effect of the gaps at the joints, is still clearly inferior to that of the heat insulating elements proper.
German Patent Document DE-OS 36 34 347, discloses a joint insulation for vacuum heat insulating elements which, on one side, rest flatly against a gastight, pressure-resistant wall. On the side that is opposite the pressure-resistant wall, the joints are covered and sealed with elastically deformable metal expansion sheets, are filled with pressure-resistant insulating material, and evacuated. In this manner, the insulating effect of the gaps at the joints is at least approximately as good as that of the heat insulating elements proper. This type of joint insulation is intended, however, for large-surface containers or large pipe lines which are covered by a plurality of plate-shaped, for example, rectangular vacuum heat insulating elements, whose gaps at the joints cross one another or merge into one another. Since the metal expansion sheets--at least in partial areas--must be mounted on the gaps at the joints subsequently, for example, by means of welding, good access of the joints is required on or in the structural component. This type of joint insulation causes problems with respect to manufacturing technology and finally is not useful in the case of interior insulations in relatively small containers.
It is therefore an object of the invention to provide a joining process for pipe-shaped, ring-shaped and dome-shaped vacuum heat insulating elements having axially aligned, surrounding edges, which can be used in the cryo-range as well as in the high-temperature range, which considerably reduces the energy losses in the area of the gap at the joints, which can be implemented in a simple manner and therefore can be used under industrial conditions as well as during mounting in the field, and which, among other things, may also be used for pipes and containers with a fairly small cross-section and with interior insulating.
These and other objects and advantages are achieved according to the invention, in which either only the outside walls or only the inside walls of the vacuum heat insulating elements are connected directly with one another in the edge area; the connection of the other walls takes place by way of an axially flexible surrounding bellows. All connections are vacuum-tight, so that the gaps at the joints which are enclosed on all sides can finally be evacuated. Before joining, the free edges of the bellows, which are connected with the heat insulating elements in a vacuum-tight manner, project axially. As a result, it is possible to first connect the bellows and then move the heat insulating elements into contact with each other and join them. Both joining operations, for example, in the form of welding, may be performed from the same side, for example, radially from the outside, which simplifies the operation. This feature of the invention makes possible the manufacturing of pipe lines and containers having a small cross-section and interior insulation. (That is, the bellows is disposed on the interior.) Since the surrounding bellows have a sufficient stiffness in the radial direction in order to absorb the mechanical stresses resulting from the pressure differences (evacuation, operating pressures), a filling of the gaps at the joints with pressure-resistant insulating material is not necessary, which also simplifies the process.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.